1. FIELD OF THE INVENTION
The present invention relates to a image-forming material. More particularly, the present invention relates to a image-forming material in which images are formed by a thermal head and the formed images are conversely erasable by water or moisture.
2. DESCRIPTION OF THE PRIOR ART
Conventional, reversible image-forming materials are disclosed in U.S. Pat. Nos. 3,414,423, 3,515,568, 3,560,229, 3,666,525, 4,028,118 and 3,244,548 as well as Published Unexamined Japanese Patent Application KOKAI No. 191190/83.
Images formed by the method disclosed in U.S. Pat. No. 4,028,118 are irrelevant to fixed images because these images change depending upon temperature.
According to the method described in U.S. Pat. No. 3,560,229, development of color formed by a color forming composition, erasure and/or permanent fixation can be controlled in the presence of heat or water by incorporating specific organic solvents in the composition (column 1, lines 64 to 68). The organic solvents used in this method are glycols, glycol ethers, halogenated biphenyls or biphenyl ethers, aromatic or aliphatic ester type plasticizers and other solvents having a low vapor pressure.
A heat-sensitive copying sheet comprising crystal violet lactone, gallic acid, acetanilide, styrene-butadiene copolymer and toluene is disclosed in U.S. Pat. No. 3,666,525 (column 8, lines 52 to 61). Images formed on this sheet are erased upon contact with water and this heat-sensitive composition requires the addition of a substance that melts upon heating.
U.S. Pat. Nos. 3,414,423 and 3,515,568 are directed to methods for re-use of heat-sensitive copying materials by erasing images formed on the materials. According to these methods, colored complexes of p-quinone compounds and dihydrobenzene compounds are disclosed upon application of certain organic solvents or heat.
Multi-layered sheets using co-reactants of lactone color forming compounds and phloroglucinol are disclosed in U.S. Pat. No. 3,244,548.
Published Unexamined Japanese Patent Application KOKAI No. 191190/83 relates to a reversible heat-sensitive transparent film, in which images formed thereon are blue, green, red, orange and yellow. With the reversible heat-sensitive transparent film, printing at a low speed with a thermal head results in printed images on the recording material but high speed printing causes irregular printing, missing or incompleteness in images, etc., further resulting in running failure of a thermal head. It is considered to be because the temperature of a recording layer coated on a support must be elevated for a short period of time in high speed printing and as a result, the temperature of the thermal head exceeds a melting point of an overcoat layer, for example, a polystyrene resin layer to cause a so-called sticking phenomenon of adhering the overcoat layer to the surface of the thermal head.